It is known that ventricular fibrillation, an often fatal heart arrhythmia, can be terminated by the application of one or more electrical current pulses delivered to the heart through electrodes applied to the chest or implanted within the body. Since the first use on humans of a completely implantable cardiac defibrillator in 1980, research has focussed on making continually smaller and more efficient defibrillation devices. In addition, reducing the defibrillation threshold (DFT) energy level applied to the heart by the defibrillation pulses reduces the likelihood of damaging tissue adjacent the electrodes.
A conventional implantable defibrillator includes an electrical pulse generator and an arrhythmia detection circuit coupled to the heart by a series of two or more electrodes implanted in the body. A battery power supply, and one or more charge storage capacitors are used for delivering defibrillation shocks in the form of electrical current pulses to the heart.
Currently, the primary constraint in reducing the size of an implantable defibrillator is reducing the battery size and the size of the storage capacitor(s). Accordingly, improvements in the area of implantable defibrillators have focussed in two areas: (1) more efficient defibrillation waveforms, and (2) more efficient electrode configurations and placements. Stated in other words, the primary variables that can be adjusted in the design to lower the shock strength required for defibrillation include those variables relating to the defibrillation waveform, such as duration, polarity, and waveshape, and those variables relating to the electrodes, such as materials, size, shape, and location.
An example of a development in the area of electrodes is U.S. Pat. No. 4,827,932 to Ideker et al. which relates to a pair of spaced apart epicardial implantable defibrillation patch electrodes. A respective patch electrode is attached over each of the right and left ventricles in an attempt to achieve a uniform voltage gradient throughout the entire ventricular mass.
In the area of defibrillation waveforms, U.S. Pat. No. 4,641,656 to Smits discloses a method of applying a sequence of defibrillating pulses to the heart from a series of four electrodes. Two adjacent electrodes have positive polarity and the other two electrodes have negative polarity in an attempt to concentrate defibrillation energy in the heart wall rather than through the center of the heart. Two or more such pulses are applied, with a reverse in polarity of one pair of opposing electrodes between each pulse. Another pulsing scheme is disclosed wherein the polarity of the four electrodes alternates with each adjacent electrode, and with all four electrodes used simultaneously to defibrillate the heart.
A publication abstract entitled New Waveforms and Defibrillation in Pigs: Biphasic, Sequential & Biphasic Sequential, by Jones et al. appearing in Proceedings AAMI, 25 Annual Meeting and Exposition, May 5-9, 1990, discloses several defibrillation waveforms delivered using an orthogonal four-patch electrode combination. An individual storage capacitor is provided to generate each of four pulses delivered to the heart. The results of the study described in the abstract provided that using two opposing electrodes and a biphasic pulse required the highest energy, that a sequential pulse using all four electrodes was intermediate, that the combination of the biphasic and sequential pulses provided a lower threshold, and that altering the order of pulses influenced the threshold with an orientation of one biphasic pathway followed by the second biphasic pathway being less efficacious than two sequential pulses of opposite polarity. Unfortunately, an implantable device having four capacitors, and the required charging and control circuitry for each of the four capacitors, would be relatively physically large and energy inefficient because of the residual charge remaining in each of the capacitors.
Other examples of defibrillating waveforms are disclosed in U.S. Pat. No. 4,637,397 to Jones et al., U.S. Pat. No. 4,800,883 to Winstrom, and U.S. Pat. No. 4,821,723 to Baker, Jr. et al. These patents disclose multiphasic defibrillation waveforms wherein the polarity of pulses is reversed. U.S. Pat. No. 4,768,512 to Imran relates to a high frequency truncated exponential waveform. U.S. Pat. No. 4,727,877 to Kallok discloses a transvenous lead configuration wherein a first electrical pulse is delivered to a first pair of electrodes between the right ventricular apex and the superior vena cava, and after a predetermined delay, a second pulse is delivered to a second pair of electrodes between the right ventricular apex and the coronary sinus.
Despite improvements in the art of waveforms for controlling fibrillation or tachycardia, there still exists a need to increase the energy efficiency of such waveforms while also reducing the likelihood of damage to tissue adjacent the implanted electrodes.